1. Field of the Invention
This invention relates to liquid fuel burners, in particular, adjustable atomizing orifice liquid fuel burners.
2. Description of the Prior Art
A frequently encountered problem for operators of combustion heated high temperature furnaces, such as glass melters, is the need to adjust the rate of fuel consumption in line with the production requirements, in particular, the output, of such furnaces. For example, at reduced output, firing rate must also be reduced. Within a given furnace having a fixed melting area, combustion volume, and burner location, conformance of the liquid fuel flame length, shape and momentum to the firing rate and load distribution within the furnace is essential for an efficient furnace operation. Thus, it is important to be able to adjust the flame characteristics at a given firing rate to provide efficient furnace operation. In most liquid fuel burner applications, the flame length, shape and momentum can be significantly adjusted by altering the degree of liquid fuel atomization. Altering the degree of atomization not only improves furnace thermal efficiency, but also increases both product quality and productivity. In addition, alteration of flame gas momentum prevents undesirable flame impingement upon the refractory of the furnace, excessive particulate entrainment and non-uniform temperature profiles which lead to hot spots and uneven heat distribution within the furnace.
Known methods for altering the degree of atomization to achieve desired flame characteristics, although simple, are nevertheless impractical. Such methods include replacement of fixed area atomizers or nozzles depending on the need to increase or decrease the atomizing fluid momentum with atomizers or nozzles having the appropriate flow geometry or area for the desired atomizing fluid momentum.
Another known method for altering the degree of atomization for achieving desired flame characteristics involves controlling the upstream pressure to the atomizer. Such pressure can be controlled by a limiting orifice valve upstream of the atomizer across which a pressure drop is taken, which pressure drop can be altered by opening and closing the valve. The change in upstream pressure to the atomizer results in a change in momentum of the atomizing fluid and, thus, shearing action for atomization between the atomizing fluid and the liquid fuel. However, this method also results in a change in the total flow rate of atomizing fluid which may not be desirable for certain grades of liquid fuels or for certain firing rates.
In addition, altering the degree of atomization to achieve certain desired flame characteristics either by changing atomizers or changing the upstream pressure of the atomizing fluid as discussed above is inefficient and time consuming. Both such methods require interruption of the process during the changeover of nozzles or the changes in upstream pressure depending on the desired firing rate or flame characteristics. Furthermore, specifically with respect to fixed area atomizing nozzles, conventional liquid fuel atomizers using such nozzles are generally designed to operate optimally near design operating conditions. At or near design conditions, the atomizing fluid flow rate and velocity at the atomizing section offer the greatest shearing action to the liquid fuel. The resulting atomization of liquid fuel having a specific droplet size distribution corresponds directly to the desired flame characteristics. Thus, any deviation from the design conditions, such as changes in atomizing fluid mass flow rate, pressure or temperature, results in poor atomization.
Off design firing rates of liquid fuel burner having fixed area atomizers cause other serious problems as well. For example, such operation can result in liquid fuel dripping and subsequent carbon formation or plugging of the fuel nozzle at which point the flame becomes unstable and deflects, directly impinging on furnace refractories, thereby damaging the refractories and shortening the furnace life. In addition, the improper flame length and shape resulting from such operation disturbs furnace temperature profile which in turn increases the total cost of heating the furnace load.
Finally, known burners have a single fuel injection configuration which restricts the burner applicability to a certain furnace size, firing rate and load distribution. No single nozzle geometry is capable of handling most furnace heating conditions. As a result, separate nozzle designs based on a particular heating application are required.
U.S. Pat. No. 4,201,538 teaches a large burner for liquid fuels capable of operating under both full load and partial load conditions having a fuel supply pipe concentrically disposed within an air supply pipe and partially enclosed by a sleeve carrying the air. The fuel supply pipe is enclosed by a swirl producing body in the form of a fixed blower wheel. The fuel supply pipe is also provided with a spray diffuser which is enclosed by a sleeve forming a passage around the fuel supply pipe through which spray diffuser air flows. Disposed between the swirl producing body and the air supply tube are two additional air supply pipes. A sliding link is provided on the fuel supply pipe which permits interruption of the air supply to the swirl producing body and an annular gap between the two additional air supply pipes when the burner is operated under partial load conditions.
U.S. Pat. No. 3,904,119 teaches an air/fuel spray nozzle in which fuel is directed radially outward from a central housing of the nozzle into helical passages formed between the central housing and outer wall of the nozzle. Air passing through the helical passages mixes with the fuel such that a uniformly distributed air/fuel mixture exits from the nozzle into the surrounding area.
To improve the combustion efficiency of a liquid fuel burner, U.S. Pat. No. 3,576,384, U.S. Pat. No. 3,733,169 and U.S. Pat. No. 3,700,173 all teach the use of swirled air for atomizing a liquid fuel discharged from a nozzle centrally disposed within an air supply pipe through which the swirled air is supplied. The '384 patent teaches an oil burner assembly in which combustion air first enters an air chamber in which the air is rotated and then passes through a nozzle around a fuel atomizer into a combustion chamber; the '169 patent teaches a flame retention head assembly for use in the air tube of a fuel burner using oil or gas in which turbulence in the air exiting from the air tube is produced by a spinner plate disposed within a cylindrical ring downstream of the outlet of the fuel nozzle; and the '173 patent teaches a diffuser for liquid fuel fired burners having widely spaced slots formed in a frusto-conical surface positioned in the path of the combustion air to cause the combustion air to intersect the atomized liquid fuel spray as independent streams to accomplish a more complete mixing thereof over a wider burner operating range.